1  #!/usr/bin/env python3
       2  """       turtle-example-suite:
       3  
       4          tdemo_planets_and_moon.py
       5  
       6  Gravitational system simulation using the
       7  approximation method from Feynman-lectures,
       8  p.9-8, using turtlegraphics.
       9  
      10  Example: heavy central body, light planet,
      11  very light moon!
      12  Planet has a circular orbit, moon a stable
      13  orbit around the planet.
      14  
      15  You can hold the movement temporarily by
      16  pressing the left mouse button with the
      17  mouse over the scrollbar of the canvas.
      18  
      19  """
      20  from turtle import Shape, Turtle, mainloop, Vec2D as Vec
      21  
      22  G = 8
      23  
      24  class ESC[4;38;5;81mGravSys(ESC[4;38;5;149mobject):
      25      def __init__(self):
      26          self.planets = []
      27          self.t = 0
      28          self.dt = 0.01
      29      def init(self):
      30          for p in self.planets:
      31              p.init()
      32      def start(self):
      33          for i in range(10000):
      34              self.t += self.dt
      35              for p in self.planets:
      36                  p.step()
      37  
      38  class ESC[4;38;5;81mStar(ESC[4;38;5;149mTurtle):
      39      def __init__(self, m, x, v, gravSys, shape):
      40          Turtle.__init__(self, shape=shape)
      41          self.penup()
      42          self.m = m
      43          self.setpos(x)
      44          self.v = v
      45          gravSys.planets.append(self)
      46          self.gravSys = gravSys
      47          self.resizemode("user")
      48          self.pendown()
      49      def init(self):
      50          dt = self.gravSys.dt
      51          self.a = self.acc()
      52          self.v = self.v + 0.5*dt*self.a
      53      def acc(self):
      54          a = Vec(0,0)
      55          for planet in self.gravSys.planets:
      56              if planet != self:
      57                  v = planet.pos()-self.pos()
      58                  a += (G*planet.m/abs(v)**3)*v
      59          return a
      60      def step(self):
      61          dt = self.gravSys.dt
      62          self.setpos(self.pos() + dt*self.v)
      63          if self.gravSys.planets.index(self) != 0:
      64              self.setheading(self.towards(self.gravSys.planets[0]))
      65          self.a = self.acc()
      66          self.v = self.v + dt*self.a
      67  
      68  ## create compound yellow/blue turtleshape for planets
      69  
      70  def main():
      71      s = Turtle()
      72      s.reset()
      73      s.getscreen().tracer(0,0)
      74      s.ht()
      75      s.pu()
      76      s.fd(6)
      77      s.lt(90)
      78      s.begin_poly()
      79      s.circle(6, 180)
      80      s.end_poly()
      81      m1 = s.get_poly()
      82      s.begin_poly()
      83      s.circle(6,180)
      84      s.end_poly()
      85      m2 = s.get_poly()
      86  
      87      planetshape = Shape("compound")
      88      planetshape.addcomponent(m1,"orange")
      89      planetshape.addcomponent(m2,"blue")
      90      s.getscreen().register_shape("planet", planetshape)
      91      s.getscreen().tracer(1,0)
      92  
      93      ## setup gravitational system
      94      gs = GravSys()
      95      sun = Star(1000000, Vec(0,0), Vec(0,-2.5), gs, "circle")
      96      sun.color("yellow")
      97      sun.shapesize(1.8)
      98      sun.pu()
      99      earth = Star(12500, Vec(210,0), Vec(0,195), gs, "planet")
     100      earth.pencolor("green")
     101      earth.shapesize(0.8)
     102      moon = Star(1, Vec(220,0), Vec(0,295), gs, "planet")
     103      moon.pencolor("blue")
     104      moon.shapesize(0.5)
     105      gs.init()
     106      gs.start()
     107      return "Done!"
     108  
     109  if __name__ == '__main__':
     110      main()
     111      mainloop()